As stated before, the core is extremely important. Hold the core on a vice grip. Then drill a hole in it. the hole should penetrate about halfway through the core. A drill press works great for this. Also, depending on what you use for a core, it may already have a hole drilled in it. Then, glue the core onto the shaft. You should glue the core on then cut the shaft to make sure that the shafts extend the same amount from the cores.